Regarding Knuckle Push-Ups
Although a 'regular' knuckle push-up may not be the best exercise to strengthen the wrist, there are many variations of the knuckle push-ups I use for this purpose.
I'll give you guys a couple of examples that I've found useful:
1. The "tiger-bend" knuckle push-up.
From the 'up' position, use a vertical fist (start on your first two knuckles) but keep your elbows in tight to your chest/ribs (do not flare elbows). When you go to the 'down' position, roll across your fist (towards your pinkie), then to the 'hammerfist' (the meaty part of your hand under your pinkie), wrists, and then finally drop the elbows to the ground. This is the 'down' position. Then reverse the order to get back up. This will work both your wrists and tri's pretty good.
2. The 'rolling' knuckle push-up.
Again this exercise requires you to use a 'vertical fist' starting on the first knuckle (supported slightly by the thumbs) on the 'up' postition. It goes through the regular push-up motion, but as you lower yourself you roll on your fist down to the last knuckle (pinkie) on the 'down' postition.
I use quite a few other variations of push-ups but these two are great for wrist conditioning.
Be careful! And train smart. :)
KG